If it was to get
a better composite sound (and in this case I might assume they were co-incident?) then it
is likely you would want to treat the mic's as one signal.

If one mic was
distant (or behind the cab, for instance) then it might be worth trying spreading the
mic's to see what they add to the mix. Often I'll take an omni room mic and place close
and room mic's hard L/R to get some spread. But I would mic with that in mind.

But only you know why you double mic'd the cab, so what were trying to achieve at the
time?

Added to which you always have to bear in mind the effects of combining the outputs of two
separated mics, and what phasing issues might result. They might sound great panned wide
apart in stereo, but in summed mono they might sound like the guitar is going down the
bath plughole!

There are good reasons for sometimes using two (or more) mics in
some situations, but the random rigging and mixing of two mics is rarely helpful. if you
do it, do it for a specific reason -- to achieve something that can't be obtained using a
single mic. Otherwise it is far better to choose and use one mic, placed carefully.

With the bass and drums, does the guitar
want to be fatter, or does it work better with just one of the mics? Does it want to be
more ambient, in which case, was the second mic far enough away to really contribute any
ambience or are they both close? If you do want it fatter, are the mics together providing
that, or is the phase relationship odd? Etc

Usually on my guitar sounds I do a close miked dynamic and a condenser a bit back for a
fuller sound, then blended in the same position, so the composite of the two tracks get
either blended on a new track or share the same bus, where I mix enough of each track to
taste. In my case as I usually do minimum of two guitars, these most of the time end hard
left/right in the pan.

Quote 4TrackMadman:Usually on my
guitar sounds I do a close miked dynamic and a condenser a bit back for a fuller sound,
then blended in the same position, so the composite of the two tracks get either blended
on a new track or share the same bus, where I mix enough of each track to taste. In my
case as I usually do minimum of two guitars, these most of the time end hard left/right in
the pan.

That is pretty much
what I have started doing although I had better results in the living room rather than the
small and deadened studio room.

In the studio I can only get the ambient mic
about 1 metre away from the amp but in the living room there is a point about 3 metres
away from the amp which seems to work quite well. It captures the room sound which
includes the reverb from the kitchen behind the mic.

I recently discovered a great way to mic with dynamic/condenser, by pointing both at the
same point on the speaker. Accidentally the condenser was actually facing out (opposite of
the speaker) but it got a great room tone so I let it sit that way in the mix.

I have a choice of mic's, yes, but for guitar cabs it's nothing esoteric. I can record a
good-sounding electric guitar cab almost anywhere - the room is practically
irrelevant.

A 57 up against the grille is my most used approach. But I
sometimes use an Audix D6, or an ADK S7 that have become my favourite alternatives. The
most I typically get away from the cab is a foot or two. Either way the room plays very
little part.

On rare occasions I'll add an omni LDC specifically to get the
room, but then I can be 20-30 feet away - I use this to add ambience, so phase is not an
issue at that distance.

Yeah...For me, if it's a room thing it'll be a good distance away, and I might use a fig 8
with the null pointing at the amp. I often find a 57 pointing at the ceiling is a good
approach. If there's more than one mic close up on the cab they're going to be either
absolutely phase aligned (with noise) or very close to that. I'd be looking for the
attributes of two different mics to make up a single coherent sound. Eg a royer and a 57.
I've never got anything but comb filtering from the close mic with one a couple of feet
back approach. I know some people like that but it doesn't really work for me.

The grille of an SM58 does cause some slight colouration, but not so much that it would
stop me using one if I didn't have a 57 to hand.

Try moving the mic around (or
getting someone else to move the mic around while you listen on phones, maybe?). Try
pointing at different parts of the speaker and different angles to the speaker. Moving the
mic by and inch or two back to front, or side to side, can produce pronounced
differences.

If the cabinet has multiple speakers then give each speaker a try.
With a mic in very close the subtle differences between supposedly 'identical' speakers
can be very evident.

Try the amp at different volumes. For thicker rhythm tones
I very often find that backing the amp off produces a fuller sound, whilst cranking the
amp up helps to emphasise the upper-mid range for lead work.

Don't rule out
unorthodox methods, such as miking the rear of the cabinet, hanging the mic vertically in
front of the cab, pointing the mic the wrong way, or pointing the mic at a baffle in front
of the cabinet. I've sometimes used a very heavy old mirror with a PZM attached to it -
great for jangly telecaster rhythms!

There's a lot you can do, so don't rule
anything out until you've tried it.